A very wide variety of industrial apparatus and processes are known for separating solid and liquid phase by means of dissolved-gas flotation. These involve dissolving a gas under elevated pressure in the fluid from which the substances are to be separated. After a certain residence time, the saturated mixture is decompressed to a lower pressure, which gives rise to gas supersaturation of the liquid medium and the gas no longer dissolved being liberated in the form of fine microbubbles.
After the addition of precipitation and flocculation aids, the solid substances to be separated are converted from the fluid into a floccular form, and the suspension thus already pretreated is further mixed with the decompressed fluid-gas mixture and then fed to a flotation tank. The mixture of fluid and gas bubbles is then fed to a flotation tank in which the flotation of the solid-flocculae composites to be separated takes place. Rectangular and round types of construction are known for the flotation tanks, various inflow geometries, for round flotation tanks, being employed which can in principle be distinguished as follows:
EP-A-0 442 463 describes a generic apparatus in which the decompression of the fluid-gas mixture under elevated pressure takes place outside the flotation tank in the feeder for the fluid containing the particles to be separated, and the already decompressed mixture flows in radially upwards via a feeder in the center of the bottom section of the flotation tank.
DE-C-27 13 088 and EP-B-0 228 396 describe systems in which the feeder of the fluid-gas mixture is designed in the form of a radial inflow pipe revolving in the flotation tank. In this case it is essential for the entire mixture of the fluid containing the substances to be separated and the gas bubbles to flow together into the flotation tank. This may either involve the gas-saturated fluid under elevated pressure being admixed to the main stream in advance, with decompression and bubble formation, similar to the above-mentioned EP-A-0 442 463, or decompression and bubble formation takes place at the same time as the injection into the flotation tank, the injection orifices, however, revolving in the flotation tank as mentioned above.
These known above-mentioned systems have the following drawbacks. When the gas bubble stream is mixed into the main fluid stream containing the solids to be separated, inflow turbulences cannot be avoided, which results in shear force stresses of the solids to be separated conditioned as flocculae, and thus may increase the chemicals consumption of flocculants and flocculation aids. Moreover, decompression of the gas-saturated fluid-gas mixture in the process sequence takes place significantly earlier than the flotation (i.e. floating-up of the solids to be separated with the aid of the gas bubbles), which results in non-optimal efficiency, since the gas bubbles may, even before the flotation process, agglomerate to form larger bubbles, and/or may degas. This gives rise to increased process losses. Moreover, in the installations having a central stationary inlet and infeed of the gas mixture upstream of the inlet into the flotation tank, there forms, owing to the pressure difference, an essentially vertically directed flow, which leads to seething and results in an inadequate distribution of the gas bubbles in the boundary region of the flotation tank.